Distillate fuel with improved seal swell properties

ABSTRACT

The invention provides a distillate fuel blend with improved seal swell properties comprising at least one highly paraffinic distillate fuel fraction having a mass swelling ratio less than 9% when measured according to ASTM D1414 or ASTM D471 at 50° C. and for 20 days when using an NBR (nitrile butadiene rubber) O-ring with a hardness of 70 that has been de-plasticised; and about 0.5 volume percent to about 15 volume percent of at least one component selected from a group of aromatic ethers wherein the blend exhibits a mass swelling ratio of at least 10% when measured according to ASTM D1414 or ASTM D471 at 50° C. and for 20 days when using an NBR (nitrile butadiene rubber) O-ring with a hardness of 70 that has been de-plasticised. The invention extends to the use of an aromatic ether fraction in a blend with a synthetic middle distillate fraction for the purposes of achieving seal swell characteristics more comparable with those characteristic of crude-derived middle distillate fuel product.

This invention is directed to a synthetic distillate fuel blend which has improved seal swell characteristics.

BACKGROUND OF THE INVENTION

It is well known that synthetic middle distillate fuel streams, such as Fischer Tropsch derived distillates, do not cause the same degree of swelling of the traditional elastomeric materials (such as nitrile O-rings) used in aircraft and other vehicles as does crude-derived fuel. This has significant potential to cause problems in situations where synthetic fuels would be treated as a drop-in component (e.g. Fully Synthetic Jet Fuel (FSJF). This is potentially far more problematic than where synthetic fuels are blended with crude-derived fuels to provide a Semi Synthetic Fuel (SSJF)). It has been further established that this lack of swelling can be rectified through the addition of various levels of aromatic species to the synthetic fuels. For example, U.S. Pat. No. 7,608,181 teaches the use of distillate-boiling alkylcycloparaffins and alkylaromatics in order to achieve improved seal swell behaviour in highly paraffinic Fischer Tropsch-derived distillate fuel.

Critically, these aromatic species require usage at levels that are comparable to the lower levels of aromatic species observed in crude-derived middle distillate fuel in order to achieve an analogous effect. Aromatic species in fuels are not themselves highly desirable from both an environmental and a combustion perspective. Hence the addition of generic aromatic species to synthetic middle distillate fuels may enable achieving the desired seal swell, lubricity and density properties; but is itself not inherently positive.

A means of achieving a synthetic middle distillate blend with suitable properties such as seal swell behaviour, but with reduced aromatic content is therefore much sought after.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a distillate fuel blend with improved seal swell properties comprising:

-   -   a) at least one highly paraffinic distillate fuel fraction         having a mass swelling ratio less than 9% when measured         according to ASTM D1414 or ASTM D471 at 50° C. and for 20 days         when using an NBR (nitrile butadiene rubber) O-ring with a         hardness of 70 that has been de-plasticised; and     -   b) about 0.5 volume percent to about 15 volume percent of at         least one component selected from a group of aromatic ethers         wherein the blend exhibits a mass swelling ratio of at least 10%         when measured according to ASTM D1414 or ASTM D471 at 50° C. and         for 20 days when using an NBR (nitrile butadiene rubber) O-ring         with a hardness of 70 that has been de-plasticised.

The highly paraffinic distillate may have a mass swelling ratio less than 6.5% when measured according to ASTM D1414 or ASTM D471 at 50° C. and for 20 days when using an NBR (nitrile butadiene rubber) O-ring with a hardness of 70 that has been de-plasticised.

The distillate fuel blend may comprise about 0.5 volume percent to about 10 volume percent of at least one component selected from the group of aromatic ethers.

The distillate fuel blend may comprise about 0.5 volume percent to about 8 volume percent of at least one component selected from the group of aromatic ethers.

The distillate fuel blend may comprise about 0.5 volume percent to about 6 volume percent of at least one component selected from the group of aromatic ethers.

The distillate fuel blend may exhibit a mass swelling ratio of at least 12% when measured according to ASTM D1414 or ASTM D471 at 50° C. and for 20 days when using an NBR (nitrile butadiene rubber) O-ring with a hardness of 70 that has been de-plasticised.

The distillate fuel blend may exhibit a mass swelling ratio of at least 14% when measured according to ASTM D1414 or ASTM D471 at 50° C. and for 20 days when using an NBR (nitrile butadiene rubber) O-ring with a hardness of 70 that has been de-plasticised.

The distillate fuel blend may exhibit a mass swelling ratio of at least 12% when measured according to ASTM D1414 or ASTM D471 at 23° C. and for 20 days when using an NBR (nitrile butadiene rubber) O-ring with a hardness of 70 that has been de-plasticised.

The distillate fuel blend may exhibit a mass swelling ratio of at least 10% when measured according to ASTM D1414 or ASTM D471 at 23° C. and for 20 days, under switch loading conditions, when using an NBR (nitrile butadiene rubber) O-ring with a hardness of 70 that has been de-plasticised.

The distillate fuel blend may have a total aromatic content of less than 8 mass %.

According to a second aspect of the invention, there is provided the use of an aromatic ether fraction in a blend with a synthetic middle distillate fraction for the purposes of achieving seal swell characteristics more comparable with those characteristic of crude-derived middle distillate fuel product.

The synthetic middle distillate fraction is highly paraffinic.

The synthetic middle distillate fraction may be a jet fuel or kerosene fraction.

The synthetic middle distillate fraction may be derived, or partly derived from Fischer Tropsch product.

The aromatic ether fraction may comprise a single aromatic ether species.

The aromatic ether fraction may comprise a combination of aromatic ether species.

The aromatic ether fraction may comprise a range of aromatic ether species such that the fraction does not have a single boiling point, but rather is characterised by a boiling point range.

The aromatic ether fraction may have a boiling point or boiling point range that lies in the middle distillate boiling point range.

The aromatic ether fraction may have a boiling point or boiling point range that lies in the kerosene boiling point range.

The aromatic ether fraction may have a boiling point or boiling point range that lies between 140° C. and 320° C. It may have a boiling point (or range) that lies between 150° and 320° C. It may have a boiling point (or range) that lies between 150° and 280° C.

In one embodiment, the aromatic ether fraction may be comprised of an aromatic ring that is entirely comprised of carbon atoms.

In one embodiment, the aromatic ether fraction may be comprised of an ether species that has a phenyl group.

In one embodiment, the aromatic ether fraction may be comprised of an ether species that has a benzyl group where R′ is either a methyl or an ethyl group.

In one embodiment, the aromatic ether fraction may be comprised of an ether species that has a phenyl group and an ether species that has a benzyl group where R′ is either a methyl or an ethyl group.

BRIEF DESCRIPTION OF FIGURES

FIG. 1: The swelling behaviour of highly paraffinic Fischer Tropsch-derived kerosene

FIG. 2: The swelling behaviour of as-received O-ring samples exposed to blends of SPK with various aromatic additives

FIG. 3: The swelling behaviour of de-plasticised O-ring samples in SPK +2%, 4% and 8% dibenzyl ether

FIG. 4.1: Switch loading experiment between Jet A-1 and SPK conducted on de-plasticised O-rings. (The swelling curves for neat Jet A-1 and SPK are provided for reference.)

FIG. 4.2: Switch loading experiment conducted on de-plasticised samples with graphs of O-rings exposed to Jet A-1 and SPK+8% toluene. (The swelling curves for neat Jet A-1 and SPK+8% toluene are provided for reference.)

FIG. 4.3: Switch loading experiment conducted on de-plasticised samples with graphs of O-rings exposed to Jet A-1 and SPK+4% dibenzyl ether. (The swelling curves for neat Jet A-1 and SPK+4% dibenzyl ether are provided for reference.)

DETAILED DESCRIPTION OF THE INVENTION Aromatic Ether Fraction

The species of this invention comprising the aromatic ether fraction is defined as an ether moiety attached to an aromatic ring. Examples of such suitable aromatic ethers include anisole, benzyl methyl ether and dibenzyl ether.

The ether moiety may contain no carbon atoms connecting the oxygen atom of the ether moiety to the aromatic ring (such as anisole); or it may contain a single carbon atom or multiple carbon atoms connecting the oxygen atom and aromatic ring (such as benzyl methyl ester). The ether moiety can also function as a bridging chain between multiple aromatic rings.

The aromatic moiety may therefore be a phenyl group, where the structure could be expressed in the general form as:

where R can be an alkyl or aryl group;

Or the aromatic moiety may be a benzyl group, where the structure could be expressed in the general form as:

where R′ can be an alkyl group and R″ can be an alkyl or aryl group;

The aromatic ether fraction may have a boiling point or boiling point range that lies between 140° C. and 320° C. It may have a boiling point (or range) that lies between 150° and 320° C. It may have a boiling point (or range) that lies between 150° and 280° C.

In one embodiment, the aromatic ether fraction may be comprised of an aromatic ring that is entirely comprised of carbon atoms.

In one embodiment, the aromatic ether fraction may be comprised of an ether species that has a phenyl group.

In one embodiment, the aromatic ether fraction may be comprised of an ether species that has a benzyl group where R¹ is either a methyl or an ethyl group.

In one embodiment, the aromatic ether fraction may be comprised of an ether species that has a phenyl group and an ether species that has a benzyl group where R¹ is either a methyl or an ethyl group.

Paraffinic Fraction

The synthetic middle distillate fraction is highly paraffinic. It can be derived from the Fischer Tropsch process (a CTL, GTL or XTL process), or derived from biological sources—for example, hydrogenated vegetable or animal oil.

The synthetic middle distillate fraction may be a kerosene fraction.

Where it is derived from a Fischer Tropsch process, this fraction can be defined as Synthetic Paraffinic Kerosene (SPK). Table 1 illustrates the general properties of two SPK fuels suitable for use in this invention compared to crude-derived product Jet A-1. SPK denotes a paraffinic CTL kerosene; and SPK-g denotes a paraffinic GTL kerosene.

TABLE 1 Properties of SPK fuels Property Limits Jet A-1 SPK SPK-g Composition Aromatics, v % 8.0*-25** 19 1 0 n-Paraffins, m % — 20 2 23 iso-Paraffins, — 26 87 76 m % cyclic-Paraffins, <15^(#) 31 10 1 m % Volatility Density @ 771-836  800 765 735 15° C., kg/m³ Thermal Stability Filter pressure 25 (max) 0 0 0 drop, mm Hg Tube deposit   <3 ^($) <1(275° C.) <1(325° C.) <1(325° C.) rating *Minimum specification applicable to SSJF and FSJF. There is no minumum specification for petroleum-derived Jet A-1 **Maximum specification for Jet A-1, SSJF and FSJF ^(#)Maximum specification applicable to FSJF. There is no maximum specification for petroleum-derived Jet A-1 ^($) Temperature of tube deposit rating dependent on fuel, 275° C. for petroleum-derived, 325° C. for synthetically derived

Blending Process

The effects of various additives were investigated by making solutions of the respective blending components in SPK. All solution blends were prepared by volume according to standard laboratory practice.

Swelling Characterisation Methods

-   1. Method for the Determination of Mass Swelling Ratio (Q %)

ASTM method D1414 (Standard Test Method for Rubber O-rings) and D471 (Standard Test Method for Rubber Property-Effect of Liquids) contain the base methods for the solution exposure experiments.

The static gravimetric method was conducted as follows:

-   -   the initial mass of the samples was recorded     -   the samples were then placed in the specified solvent     -   at specified times the samples were removed from the solvent and         blotted dry before weighing     -   finally the samples were returned to the solvent.

The containers were placed inside a closed box in order to eliminate any influence of light exposure. Although this is not specified in the ASTM methods it was, however, deemed important for the long exposure treatments, continuing for durations longer than 100 days. The procedure was continued until the samples had reached equilibrium, i.e. until a change of mass was no longer observed. From the data obtained the mass swelling ratio (Q %) was determined as a function of time. Q % is given by the equation below.

${Q\mspace{14mu} \%} = {100 \times \frac{M_{t} - M_{o}}{M_{o}}}$

-   -   where:     -   M₀ mass before swelling     -   M_(t) mass at time, t

For certain experiments the O-rings were removed from solution after equilibrium had been reached and exposed to air for 1 day, allowing the bulk of the remaining fuel in the polymer to evaporate. The sample was then placed in the vacuum oven at 50° C. for 5 days. Final mass measurements after solvent extraction where then made to allow the extent of mass loss due to plasticiser extraction to be determined.

-   2. Method for the Determination of Volume Swelling Ratio (%)

ASTM method D1414 (Standard Test Method for Rubber O-rings) and D471 (Standard Test Method for Rubber Property-Effect of Liquids) contain the base methods for the solution exposure experiments.

In ASTM D1414, volume changes may be observed using a hand micrometer of O-ring diameter according to which the cross-sectional diameter is measured at four points equally distributed around the circumference. This method was found to be inaccurate; so optical microscopy was used.

A similar experimental procedure was used to that of gravimetric method. However, the change in O-ring volume was determined using an optical microscope at 40× (optical) magnification. The average diameter of the O-rings were measured from six points equally distributed around the sample by taking images of the O-rings undergoing solution treatment. Each diameter reading was determined using the circular measurement option of the ShuttleRix® software. This allowed for the inside diameter (i.d.) and outside diameter (o.d.) to be measured and thus the thickness of the sample to be determined at one of the six measuring points. The change in cross-sectional area can be used to determine the change in volume of the sample using the equation:

${R\mspace{14mu} \%} = {100\left( {\left( \frac{d_{f}}{d_{o}} \right)^{3} - 1} \right)}$

-   -   where:     -   d_(o)=initial cross-sectional diameter     -   d_(f)=cross-sectional diameter at time, t

-   3. As-Received and De-Plasticised O-rings

The nitrile butadiene rubber (NBR) O-rings used in this study were supplied by Bearing Man Ltd (Johannesburg, South Africa). These had an inside diameter of 20 mm, a 2.5 mm cross-sectional diameter and a Shore-A hardness of approximately 70. Later measurements were performed on smaller O-rings of inside diameter 4.2 mm and cross-sectional diameter 1.9 mm and Shore-A hardness 70. These were also supplied by Bearing Man Ltd.

Extractable polymeric additives, such as plasticisers and curatives complicate the interpretation of swelling data obtained from as-received samples undergoing solution treatment, since the measured data is the result of solvent entering the polymer and the extraction of additives. It was found that solvents/fuel blends that show seal swell potential will remove plasticisers from the NBR O-rings. For this reason, O-ring conditioning with CH₂Cl₂ was employed in certain circumstances to remove plasticisers. After the conditioning process it was determined that 10.0%±0.2% mass loss occurred which was attributed to extractible additives in the O-ring samples under investigation. This value was supported by TGA (Thermal Gravimetric Analysis) results. (Note that this value is highly dependent on the polymeric component being used.).

The method used for generating the deplasticised O-rings was as follows:

20 O-rings were placed in 800 mL of solvent (CH₂Cl₂) for 3 days at a constant temperature of 23° C. The solvent was then replaced with fresh supply and samples were left for an additional 3 days. After the extraction of plasticiser the solvent in the O-ring was evaporated off by allowing the samples to air dry for 1 day, followed by vacuum extraction at 50° C. for 5 days at −0.80 bar.

The invention will now be illustrated by the following non-limiting examples:

EXAMPLES Example 1 Comparative Example Base Cases

As a base case, the swelling behaviour of two samples of highly paraffinic Fischer Tropsch-derived kerosene was determined. They are designated SPK (an FT coal-derived isomerised kerosene) and SPK-g (an FT gas-derived kerosene that contains less isomerised paraffin than does SPK (see Table 1)). FIG. 1 shows the swelling behaviour of these samples over time, in tests conducted on as-received NBR O-rings at 50° C. The dramatic decrease in seal swelling as a result of plasticiser loss with these samples is easily observed.

Table 2 shows the swelling behaviour of blends of Jet A-1 and SPK (showing the effect of SSJF composition) in a series of experiments using de-plasticised O-rings exposed to the blends at 50° C. As more Jet A-1 is incorporated, the swelling behaviour increases. Note that, whilst the volume change seems positive for the pure SPK sample in this case, this measurement is made after removal of the plasticiser (which reduces the volume by 12.5%), so the net change is actually negative.

TABLE 2 Swelling of de-plasticised O-rings exposed to blends of Jet A-1 and SPK Volume change at Volume change at Mass change at equilibrium (%) - equilibrium (%) - % Jet A-1 equilibrium (%) large O-rings small O-rings 0 (neat SPK) 4.8 (0.1)  8.8 (0.3)  7.7 (0.9) 25 7.0 (0.1) 11.9 (0.3) 11.0 (0.6) 50 9.0 (0.0) 13.9 (0.3) 13.1 (0.4) 75 10.8 (0.1)  17.1 (0.1) 16.1 (0.5) 100 (neat 14.4 (0.1)  20.7 (0.2) 20.1 (0.6) Jet A-1) The values in brackets are the standard deviations of the mean. The standard deviations for volume changes of small O-rings are larger because the contribution of flash to projected area is larger.

Example 2

A range of various aromatic additives (anisole, dibenzyl ether, toluene and benzyl alcohol (designated BzOH) were tested in blends with SPK on as-received NBR O-rings at 50° C. in order to ascertain their effect on seal swell. FIG. 2 shows the swelling behaviour of O-rings exposed to these blends over time under static conditions. A sample of SPK blended 50/50 with Jet A-1 representative of a commercially approved SSJF was also included for comparison purposes. It is clear that the aromatic ether samples are significantly more efficacious in achieving seal swell than the other two aromatic species tested. It is also evident that at an 8% additive level, both aromatic ether species provide seal swell behaviour far in excess of what is observed for SSJF.

Example 3 Effect of Additive/Solvent Concentration on Swelling

A range of concentrations of dibenzyl ether additive in SPK was tested on de-plasticised O-rings at 50° C. in order to ascertain the level of the blending component required to produce a similar swell to that seen in samples exposed to Jet A-1. (The Jet A-1 sample used in these experiments contained approximately 18% aromatics.)

FIG. 3 shows the swelling of de-plasticised O-ring samples in SPK+2%, 4% and 8% dibenzyl ether. Table 3 shows the key results in tabulated form. The effect on mass swelling ratio at various concentrations (as shown in FIG. 3) indicates that seal swell levels comparable to those observed for Jet A-1 (the red dotted line on the graph) can be easily achieved at levels of just 5.3 volume % dibenzyl ether.

TABLE 3 Key data showing impact of dibenzyl ether additive levels on swelling Calculate average Average mass uptake volume change at Solution at equilibrium (%) equilibrium (%)* Jet A-1 14.4 (0.1) 20.7 SPK  4.8 (0.1) 8.9 SPK + 1% Dibenzyl ether  6.7 (0.1) 11.2 SPK + 2% Dibenzyl ether  8.5 (0.1) 13.4 SPK + 4% Dibenzyl ether 12.2 (0.1) 17.9 SPK + 8% Dibenzyl ether 19.3 (0.1) 26.6** The values in brackets are the standard deviations of the mean. N = 3 *includes the volume increase due to SPK component **Measured change was 26.4

Example 4 Temperature Effects on Swelling

The effect of temperature on the seal swell ability of SPK additised with two different types of aromatic species was assessed. This experiment was of interest because of the requirement that these additives be able to function effectively across the temperature range of the operating environment where the O-rings are to be used. The two blends were SPK+0.5 vol % benzyl alcohol (BzOH) and SPK+8 vol % dibenzyl ether. These experiments were performed on statically treated O-ring samples at temperatures of 23° C. and 50° C. respectively. Swelling was measured until an equilibrium state was reached

FIG. 4.1 shows the effect of temperature on the swelling of O-rings exposed to SPK+0.5% BzOH (referred to as BzA in the figure). It is clear that O-rings treated at 23° C. reach a greater equilibrium mass swelling ratio (which is in excess of the samples exposed to Jet A-1) than do the rings treated at 50° C. FIG. 4.2 shows the effect of temperature on the swelling of de-plasticised O-rings exposed to a blend of SPK+8% dibenzyl ether. It is clear that, in the case of dibenzyl ether, the behaviour at ambient and 50° C. conditions was far more consistent than was the case for benzyl alcohol.

Example 5 Switch Loading Experiment

An investigation was conducted into the swelling behaviour that occurs when switching between petroleum-derived fuel and synthetic fuels, known as switch loading. These experiments were done in order to represent more realistic conditions that an O-ring may face in service should the fuel chemistry be changed.

Initial switch loading experiments were run on statically treated O-ring samples by switching solvents every 7 days from Jet A-1 to SPK, and recording the mass changes. This was followed by experiments using switching between Jet A-1 sample and blends of SPK with one of two additive components—toluene (at 8 vol %) and dibenzyl ether (at 4 vol %). The effect of switching fuel types was hence monitored as Q % (mass swelling ratio) over time. These experiments were performed using deplasticised O-rings and conducted at room temperature. In all these switching experiments, the samples were exposed to the Jet A-1 sample first.

FIG. 5.1 shows the experimental results of switching between Jet A-1 and pure SPK. (The swelling curves for Jet A-1 and pure SPK are included for comparison.) The swell is contained between upper and lower limits defined by the swell behaviour in the respective solvents when no switching occurs.

FIG. 5.2 shows the experimental results of switching between Jet A-1 and a blend of SPK+8 vol% toluene. (As before, the swelling curves for neat Jet A-1 and the SPK+8 vol % toluene blend are included for comparison.) FIG. 5.3 shows the analogous experiment switching between Jet A-1 and a blend of SPK+4 vol % dibenzyl ether.

The blend of SPK+4 vol % dibenzyl ether shows far less change during the switching experiment than does the blend of SPK+8 vol% toluene. At equilibrium, the swelling of the SPK+8 vol % toluene blend is clearly less than that obtained for the SPK+4 vol % dibenzyl ether blend.

Example 6 Impact of Various Additive Species on Seal Swell (Measured as Volume % Change) at Higher Temperatures

For the purpose of further quantifying the effect on seal swell of various classes of additive, SPK was blended with various aromatic additives according to the concentrations described in Table 4 below. These blends were then run on statically treated O-ring samples. The swelling of de-plasticised O-rings was measured at 50° C. until an equilibrium state was reached. This swelling was then characterised as a volume % change.

TABLE 4 Solvent properties, additive levels and experimental results showing solvent effects at higher temperatures. % Volume Swelling Boiling Concentration Mass swelling change (at efficacy Solvent Formula point (° C.) (v/v) ratio at 50° C. 50° C.) (%) Jet Fuel product Jet A-1 100% 13.9 20.5% SPK 100% 6.0 9.1% Cycloparaffins Decalin C₁₀H₁₈ 187 8% in SPK 6.6 11.6% 0.17% Aromatics Benzene C₆H₆ 80 8% in SPK 11.1 16.8% 0.98% Toluene C₇H₈ 111 8% in SPK 9.5 15.4% 0.79% Cumene C₉H₁₂ 151 8% in SPK 7.8 13.2% 0.51% p-Cymene C₁₀H₁₄ 177 8% in SPK 7.5 12.2% 0.39% Tetralin C₁₀H₁₂ 207 8% in SPK 9.5 15.6% 0.81% Methyl C₁₁H₁₀ 240 8% in SPK 8.0 16.2 1.64% Napthalene Aromatic ethers Anisole C₇H₈O 154 8% in SPK 14.8 22.1% 1.63% Dibenzyl Ether (C₆H₅CH₂)₂O 158 8% in SPK 23.3 26.1% 2.13% Other aromatic oxygenates Furan C₄H₄O 31 8% in SPK 10.8 16.4% 0.94% Benzyl Alcohol C₆H₅CH₂OH 158 0.5% in SPK   8.6 12.4% 6.60% Note that these experiments were carried out on de-plasticised O-rings, so they do not represent the net volume or mass change from “as received” O-rings where the plasticiser is removed by the solvent/additive. (The de-plasticised O-ring has already seen a mass loss of approximately 10.0% and a volume change of approximately 12.5% due to the removal fo the plasticiser.) Table 4 shows that the aromatic ethers outperform the other additives with very high swelling values.

In order to calculate a swelling efficacy factor, the effective volume change (due to the use of additive solvent itself) was calculated by subtracting the volume change that occurs with neat SPK; and then dividing this value by the concentration of the additive. This gives a value indicating the capacity of the additive to improve swelling, normalised by the amount of additive that was used.

When these values are normalised by the additive concentration, the aromatic ethers still score very highly, especially against the other aromatic species. 

1-16. (canceled)
 17. A distillate fuel blend with improved seal swell properties comprising: at least one synthetic paraffinic middle distillate fuel fraction derived from a Fischer-Tropsch process or a biological source, the synthetic paraffinic middle distillate fuel fraction having a mass swelling ratio less than 9% when measured according to ASTM D1414 or ASTM D471 at 50° C. and for 20 days when using a nitrile butadiene rubber O-ring with a hardness of 70 that has been de-plasticised; and about 0.5 volume percent to about 15 volume percent of at least one component selected from the group consisting of aromatic ethers.
 18. The distillate fuel blend of claim 17, wherein the synthetic paraffinic middle distillate fuel fraction has a mass swelling ratio less than 6.5% when measured according to ASTM D1414 or ASTM D471 at 50° C. and for 20 days when using a nitrile butadiene rubber O-ring with a hardness of 70 that has been de-plasticised.
 19. The distillate fuel blend of claim 17, wherein the distillate fuel blend comprises about 0.5 volume percent to about 10 volume percent of at least one component selected from the group consisting of aromatic ethers.
 20. The distillate fuel blend of claim 17, wherein the distillate fuel blend comprises about 0.5 volume percent to about 6 volume percent of at least one component selected from the group consisting of aromatic ethers.
 21. The distillate fuel blend of claim 17, wherein the distillate fuel blend has a total aromatic content of less than 8 mass %.
 22. The distillate fuel blend of claim 17, having seal swell characteristics comparable with those characteristic of crude-derived middle distillate fuel products.
 23. The distillate fuel blend of claim 22, wherein the synthetic paraffinic middle distillate fuel fraction is highly paraffinic.
 24. The distillate fuel blend of claim 22, wherein the synthetic paraffinic middle distillate fuel fraction is a jet fuel or a kerosene fraction.
 25. The distillate fuel blend of claim 22, wherein the synthetic paraffinic middle distillate fuel fraction is derived or partly derived from Fischer-Tropsch product.
 26. The distillate fuel blend of claim 22, wherein an aromatic ether fraction of the distillate fuel blend comprises a single aromatic ether species.
 27. The distillate fuel blend of claim 22, wherein an aromatic ether fraction of the distillate fuel blend comprises a combination of aromatic ether species.
 28. The distillate fuel blend of claim 22, wherein an aromatic ether fraction of the distillate fuel blend has a boiling point or boiling point range that lies in a middle distillate boiling point range. 